[1] Harvard Univ, Massachusetts Gen Hosp, Div Obstet Anesthesia, Dept Anesthesia Crit Care & Pain Med,Med Sch, Boston, MA 02114 USA
[2] Ctr Dis Control & Prevent, Div Reprod Hlth, Natl Ctr Chron Dis Prevent & Hlth Promot, Atlanta, GA USA
[3] Ctr Dis Control & Prevent, Div Heart Dis & Stroke Prevent, Natl Ctr Chron Dis Prevent & Hlth Promot, Atlanta, GA USA